1. SANTOSH CHEMISTRY DEPT
Chemical kinetics
SANTOSH
CHEMISTRY DEPT

2. Chemical Kinetics
Chemical kinetics - speed or rate at which a reaction occurs
How are rates of reactions affected by
Reactant concentration?
Temperature?
Reactant states?
Catalysts?

3. The Instantaneous Reaction Rate
Consider the following reaction
A + B  C
Define the instantaneous rate of consumption of reactant A, A

4. Reaction Rates and Reaction Stoichiometry
Look at the reaction
O3(g) + NO(g) ® NO2(g) + O2(g)

5. Another Example
2 NOCl (g)  2 NO + 1 Cl2 (g)
WHY? 2 moles of NOCl disappear for every 1 mole Cl2 formed.

6. The General Case a A + b B ® c C + d D
rate = d[A] = -1 d[B] = +1 d[C] = +1 d[D]
a dt b dt c dt d dt
Why do we define our rate in this way?
removes ambiguity in the measurement of reaction rates
Obtain a single rate for the entire equation, not just for the change in a single reactant or product.

7. Alternative Definition of the Rate
Rate of conversion related to the advancement of the reaction, .
V = solution volume

8. An Example Examine the following reaction
2 N2O5 (g)  4 NO2 (g) + O2 (g)
N2O5
NO2 O2
Initial nø
Change
-2
+4 +
Final
nø - 2 4 

9. Note – constant volume system
The N2O5 Decomposition
Note – constant volume system

10. The Rate Law
Relates rate of the reaction to the reactant concentrations and rate constant
For a general reaction
a A + b B + c C ® d D + e E

11. Rate Laws (Cont’d)
The only way that we can determine the superscripts (x, y, and z) for a non-elementary chemical reaction is by experimentation.
Use the isolation method (see first year textbooks).

12. The Concentration Dependencies of the Species
The amounts of the reactants are related to the reaction rates as follows.

13. Temperature Dependence of Reaction Rates
Reaction rates generally increase with increasing temperature.
Arrhenius Equation
A = pre-exponential factor
Ea = the activation energy

14. Reactions Approaching Equilibrium
Examine the concentration profiles for the following simple process.
A ⇌ B

15. Approaching Equilibrium
Calculate the amounts of A and B at equilibrium.

16. The Equilibrium Condition
At equilibrium, vA,eq = vB,eq.

17. Elementary steps and the Molecularity
Kinetics of the elementary step depends only on the number of reactant molecules in that step!
Molecularity  the number of reactant molecules that participate in elementary steps

18. The Kinetics of Elementary Steps
For the elementary step
unimolecular step
For elementary steps involving more than one reactant
a bimolecular step

19. The Kinetics of Elementary Steps
For the elementary step
unimolecular step
For elementary steps involving more than one reactant
a bimolecular step

20. The Reaction “Yield”
We can calculate the amount of material produced from the competing reactions.
kJ = the rate constant for the reaction J.

21. Activated Complex Theory
Consider the following bimolecular reaction
Presume that the reaction proceeds through the transition state?

22. The Activated Complex
The species temporarily formed by the reactant molecules – the activated complex.
A small fraction of molecules usually have the required kinetic energy to get to the transition state
The concentration of the activated complex is extremely small.

23. Transition State Theory (TST)
TST pictures the bi-molecular reaction proceeding through the activated complex in a rapid-pre-equilibrium.

24. TST (II)
From the thermodynamic equilibrium constant for the formation of AB↕

25. TST (IV)
Assume that we can obtain the Gibbs energy of activation from K↕.
The Eyring Equation!

26. TST (V)
Hence, substituting for the Gibbs energy of activation in terms of the enthalpy and entropy of activation.

27. TST (VI)
Expressing the enthalpy of activation in terms of the activation energy.

28. TST (VII)
The final result for a bimolecular gas phase reaction.

29. TST (VIII)
The final result for a unimolecular gas phase reaction (or any solution reaction).

30. THANKS